Identifying the genetic basis of large-embryo mutants of barley affected at the Lys3 locus.

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Grain quality in cereals is determined by the composition and relative sizes of the three main components of the grain: the endosperm, embryo (or germ) and the husk. The endosperm is starch-rich and it provides the bulk of dietary calories. In contrast, the embryo accounts for a small proportion of grain weight but it is nutrient-rich. The embryo contains several essential nutrients that are lacking or less abundant in the other grain components. The relative sizes of the embryo and endosperm appear to be tightly regulated in cereals but there are a few examples of grains with larger-than-normal embryos and consequently, improved grain quality. A good example is the giant embryo mutant of maize. This was originally selected for high oil content and consequently, improved nutritional value for animal feed. It was shown subsequently that the improved nutritional properties were due to the larger-than-normal embryos.In barley, we have shown that a locus, Lys3 that controls grain lysine content also controls embryo size. Barley mutants affected at the Lys3 locus have embryos that are approximately twice the mass of normal embryos. Four independent mutant alleles of Lys3 have been identified in different labs and in different genetic backgrounds. As with the giant embryo mutants of maize, lys3 mutants of barley have also shown improved nutritional value as animal feed. Unfortunately, their uptake has been limited by reduced grain yield caused by reduced endosperm size and therefore low grain weight.In addition to the affects of lys3 mutations on grain quality and organ size, we recently discovered an unexpected side-effect on the efficiency of barley transformation. At present, very few barley cultivars other than Golden Promise are susceptible to transformation mainly due to the inability of the excised immature embryos to regenerate shoots in culture. One of the lys3 mutants consistently gives transformation efficiencies equivalent to those of Golden Promise.Our preliminary work suggests that although there is an orthologue of the giant embryo gene of maize in barley, it lies on a different chromosome from the Lys3 locus. Furthermore, no genes similar to the giant embryo gene have been found in the region of the barley genome containing Lys3 suggesting that the mechanism determining embryo size in the lys3 mutants may be novel. To understand the biological basis of the control of embryo size in barley and in the long term, to manipulate this most effectively and efficiently in cereal species generally, we need to identify the Lys3 gene. To do this, we propose to fine map the region of the barley genome containing Lys3. In addition we aim to transfer a lys3 mutant allele to elite barley cultivars thus generating tools for future barley improvement by genetic engineering.We expect that the benefits of this study will be 1) to further our understanding of the determination of embryo size in barley and other species, 2) to suggest strategies for further nutritional enhancement of barley, retaining the positive aspects of the lys3 phenotype but without the detrimental effects on endosperm size and therefore yield, and 3) to understand of the role of lys3 in influencing shoot regeneration from embryos in culture and to extend the range of genotypes of barley and ultimately, of other species that are amenable to transformation.